Easy Debian Chroot is a project focused on getting a Debian filesystem alongside OS2008, and being able to run Debian applications from that filesystem.

Deblet is a project focused on a bootable Debian system that runs on its own; it can be combined with some stuff from Easy Debian Chroot to use the same filesystem for chrooting and booting.

Your other questions seem to be more directed at Deblet-like usage; as I'm still using a hybrid of Beta3 (an earlier project in the same vein as Deblet), with some chroot stuff borrowed from, contributed to, and just plain different from Easy Debian Chroot, I'm unable to give good answers. (But I will point out that KDE, GNOME, and XFCE will all slow anything down noticeably.)